If you don't reply to an RSVP (that already has a stamp on it AND an address) then you DONT have a seat in the wedding. You can't just show up unexpected. Have a hard time putting the envelope in the mailbox? Just go to the link on the invitation and rsvp online! 
You also cannot bring your distant cousin, and her boyfriend as well. If the invitation is addressed to you and a spouse or a date, you bring one, not your whole freaking family, and if you think it's the polite thing to do, AT LEAST RSVP to the damn wedding.
It's not a bar or a huge social event, It's a formal wedding, you can't just bring whoever you please and their grandma.We are paying per head, not just for space, geez people.
Rant over. 